Process for sulfur production

ABSTRACT

Sulfur is produced by contacting, in a catalytic selective oxidation zone, a feed gas comprising an acid gas stream containing hydrogen sulfide in admixture with about 70 to 130 percent of the stoichiometric amount of oxygen required for conversion of hydrogen sulfide to sulfur and a recycle gas which is a portion of the gas resulting from condensing sulfur from the effluent of the catalytic selective oxidation zone with a catalyst selectively capable of oxidizing hydrogen sulfide to sulfur dioxide substantially without formation of sulfur trioxide to form a gas stream comprising hydrogen sulfide, sulfur dioxide and sulfur at a temperature between the kindling temperature of the catalyst and about 850° F. Formed sulfur is condensed from the effluent gas stream and a portion of the substantially sulfur-free effluent returned as recycle gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of application Ser. No.198,624, filed Oct. 20, 1980, now abandoned, which is aContinuation-in-Part of application Ser. No. 33,873, filed Apr. 27,1979, now U.S. Pat. No. 4,279,882.

BACKGROUND OF THE INVENTION

The modified Claus process has been widely applied for the production ofsulfur from acid gas feeds containing hydrogen sulfide in admixture withvarying amounts of carbon dioxide. The acid gas streams usually containsmall amounts of hydrocarbons ranging from methane to butane and evenhydrocarbons of higher molecular weight. All industrial Claus unitsstart with a thermal reaction zone in which air is added in thestoichiometric quantity needed to react hydrogen sulfide to sulfur bythe reaction:

    H.sub.2 S+1/2O.sub.2 →S+H.sub.2 O

In the thermal reaction zone, sulfur dioxide is formed. A portion of theformed sulfur dioxide reacts with hydrogen sulfide to form additionalsulfur. The main products of the thermal reaction zone are elementalsulfur, sulfur dioxide, unconverted hydrogen sulfide and a considerableamount of heat, which is ordinarily removed by generating steam in aheat exchanger. When hydrocarbons are present, there is also formedcarbonyl sulfide and carbon disulfide by competing reactions. The gasfrom the thermal reaction zone is cooled and sulfur condensed andremoved. The gas is reheated and passed to one or more catalytic stageswhere sulfur dioxide is reacted with hydrogen sulfide over alumina orbauxite catalyst to produce additional sulfur which is removed bycooling and condensation between catalytic stages. The catalytic sulfurforming reaction is:

    2H.sub.2 S+SO.sub.2 ⃡3S+2H.sub.2 O

In the typical straight-through Claus process, as described above, ifthe inert content of the acid gas stream (e.g. CO₂ or N₂) exceeds about50 percent by volume of the feed, the flame temperature becomesmarginally low because of the inert burden. It then becomes necessary tochange the flow diagram of the plant by either heating the air and/orthe acid gas feed, or diverting part of the acid gas feed around thethermal reaction zone.

With increasing inert content, more and more of the acid gas feed mustbe diverted, and the thermal reaction zone approaches the situation inwhich sulfur dioxide is the major product of the thermal reaction zone,with little or no sulfur being formed and with little or no unreactedhydrogen sulfide in the flame. When the inert content of the acid gasreaches 75 to 80 percent, it becomes difficult or impossible to maintaina steady flame reaction, even when all the hydrogen sulfide is convertedto sulfur dioxide.

A known method of dealing with a gas containing 80 percent or moreinerts is disclosed in my U.S. Pat. No. 3,880,986, incorporated hereinby reference. In this process, a thermal reaction is used to producesulfur dioxide from elemental sulfur formed in the process.

In still other cases, a workable flame temperature may be sustained byadding hydrocarbon gas as a fuel. This, however, significantlycomplicates control of the process, creates the danger of forming tarryproducts and discolored sulfur, and reduces the recovery of sulfur byforming water, a reaction product which is adverse to the Clausequilibrium. It also amplifies the problem of forming carbonyl sulfideand carbon disulfide, which are difficult to convert on a continuousbasis in the Claus plant.

As indicated, all of the industrially used Claus processes involve athermal reaction step where sulfur dioxide is formed alone or withsulfur, the sulfur dioxide being later reacted with hydrogen sulfide toform sulfur. With no exceptions, the heat generated by the formation ofsulfur dioxide is removed in a heat exchanger preceding the catalyticconversion stages. Thus, such plants all require a combustion chamberand a heat exchanger, which constitutes a substantial part of the costof the entire plant and which adds considerably to the problems ofcontrolling plant operation.

A need exists for an effective process for sulfur production which doesnot employ an expensive and difficult to control thermal reactor.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a novel Clausprocess which eliminates the thermal reactor, including its combustionchamber and heat exchanger, and which can be applied to the treatment ofgas streams containing from about 10 to 100 percent by volume,preferably from about 15 to 100 percent by volume, hydrogen sulfide, andmore preferably from about 20 to 100 percent by volume hydrogen sulfide.The process is particularly suited to small plants having an output of20 tons per day of sulfur or less and minimizes human supervision. Mosttypically, the feed gas stream can contain at least 5 percent by volume,preferably at least 10 percent by volume, hydrogen sulfide.

According to the process of the present invention, sulfur is produced bycontacting in a catalytic selective oxidation zone a feed gas streamcomprising an acid gas stream containing hydrogen sulfide in admixturewith oxygen, normally supplied as air, and present in an amount of fromabout 70 to 130 percent of the stoichiometric amount required foroxidation of the hydrogen sulfide to elemental sulfur, and a recycle gaswhich is a portion of a residual gas comprising hydrogen sulfide andsulfur dioxide resulting from condensation of sulfur from the effluentof the catalytic selective oxidation zone, with a selective oxidationcatalyst capable of selectively oxidizing hydrogen sulfide to sulfurdioxide substantially without formation of sulfur trioxide. Astoichiometric amount of oxygen is preferably employed. Introducedhydrogen sulfide is catalytically converted to sulfur dioxide and sulfurat a temperature above the kindling temperature of the catalyst andbelow about 1000° F., preferably below 850° F. or 700° F., as controlledby the amount of recycle gas introduced to the catalytic selectiveoxidation zone. There is formed a product gas stream comprising hydrogensulfide, sulfur dioxide and sulfur. Normally, the molar ratio ofhydrogen sulfide to sulfur dioxide in the gas stream will be about 2:1.

Typically, the feed gas stream contains at least 5 percent by volume,preferably at least 10 percent by volume, hydrogen sulfide. Independentof the above limitation, the acid gas stream can contain from 10 to 100percent by volume hydrogen sulfide, preferably from about 15 to 100percent by volume hydrogen sulfide.

The product gas stream is then cooled to below the dew point of sulfurto condense sulfur and leave a cooled residual gas comprising hydrogensulfide and sulfur dioxide. A portion of the cooled residual gas isreturned as recycle gas to the catalytic selective oxidation zone. Thebalance may be incinerated, sent to a tail gas treating operation orprior to tail gas treating and/or incineration, heated and passed to aClaus conversion zone to form additional sulfur.

The amount of gas recycled will range from about 0.1 to about 10 molesper mole of acid gas feed, depending upon the hydrogen sulfide contentof the acid gas feed with the amount of recycle gas being directlyproportional to the hydrogen sulfide concentration.

The upper temperature of 1000° F., and preferably 850° F., is to preventthe formation of tarry products from hydrocarbons in the acid gas feedwhich can lead to deactivation of the catalyst by coking, overheating orsulfation. Where mild steel is the material of construction, as well aswhen olefins and/or paraffins containing at least three carbon atoms arepresent in the feed, it is preferred to limit reaction temperature toabout 850° F., or preferably 700° F.

As indicated, the catalyst used in the selective oxidation zone is onecapable of completely reacting all input oxygen selectively withhydrogen sulfide to form sulfur dioxide without appreciable formation ofsulfur trioxide. This is essential to preclude unreacted oxygen andsulfur trioxide contacting the catalyst used in any subsequent Clauscatalyst zone to which the gas which is not recycled may be fed. Thecatalyst should also have the desired characteristic of beingsubstantially incapable of oxidizing hydrogen, methane and carbonmonoxide.

The presently preferred catalysts comprise a vanadium oxide and/orsulfide on a non-alkaline porous refractory oxide. Typical catalystscomprise from about 1 to about 30 percent by weight, preferably fromabout 5 to about 15 percent by weight, of a vanadium compound calculatedas the oxide and normally in the oxide and/or sulfate state, preferablyas V₂ O₅, deposited on the non-alkaline refractory oxide support. Suchsupports include alumina, titania, silica, silica-alumina, magnesia,silica-magnesia, silica-magnesia, zirconia, silica-zirconia,silica-titania, silica-zirconia-titania, certain acid metal phosphates,acid metal arsenates, crystalline or amorphous aluminosilicate hydrogenzeolites having a silica to alumina ratio between about 4:1 and 100:1,and their mixtures. Such catalysts have kindling temperatures as low asabout 270° F., and an inlet temperature of about 270° F. to about 450°F., preferably from about 325° F. to about 400° F. is employed.

Preferably, that portion of the gas from the catalytic selectiveoxidation zone which is not recycled back to the catalytic selectiveoxidation zone is passed at a temperature from about 350° F., morepreferably 400° F. or more, to at least one additional Claus catalyticconversion zone where additional sulfur is formed by reaction ofhydrogen sulfide and sulfur dioxide over a conventional Claus catalystsuch as alumina or bauxite.

The practice of the process of this invention permits the production ofsulfur from acid gas feeds of any hydrogen sulfide concentration withoutthe use of a thermal reaction stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached drawing is a flow diagram illustrating the process of theinstant invention in its preferred form.

DETAILED DESCRIPTION

The present invention is directed to a process for production of sulfurfrom hydrogen sulfide solely by the use of catalysts and which does notinvolve thermal oxidation of hydrogen sulfide.

With reference to the attached Drawing, the acid gas feed containinghydrogen sulfide, preferably in an amount of from about 10 to 100percent by volume hydrogen sulfide, or preferably from about 15 to 100percent by volume hydrogen sulfide, and more preferably from about 20 to100 percent by volume hydrogen sulfide, in line 10 is combined withrecycle gas in line 12. The recycle gas is a portion of the product gasof reactor 14 containing a selective oxidation catalyst, as definedbelow, after sulfur removal in condenser 16. The mixture after beingheated to a temperature sufficient, after accounting for air addition,for feed to reactor 14 in direct heat exchange 18 by a suitable heattransfer fluid, is combined with a source of oxygen, typically air,introduced by line 20 to line 22, and passed to catalytic selectiveoxidation zone 14 at at least the kindling temperature of the catalystcontained therein. One or more catalytic selective oxidation zones inseries or in parallel may be employed. The net feed gas stream cancontain at least 5 percent by volume, and preferably at least 10 percentby volume, hydrogen sulfide.

Reactor 14 contains a selective oxidation catalyst. By a "selectiveoxidation catalyst", as used herein, there is meant a catalystselectively capable of oxidizing hydrogen sulfide to sulfur dioxide,substantially without formation of sulfur trioxide; is preferablyincapable of oxidizing hydrogen, methane, and carbon monoxide and isresistant to or incapable of deactivation by sulfur trioxide. Suchcatalysts include a catalyst containing vanadium in the oxide and/orsulfide state deposited on a non-alkaline porous refractory oxide.Typical catalysts comprise from about 1 to about 30 percent by weight avanadium compound calculated as the oxide and in the oxide and/orsulfide state, preferably in the form of V₂ O₅, based on the weight ofvanadium compound and support, deposited on a porous refractory oxideessentially free of alkali and alkali earth metals. The refractoryoxides may be alumina, titania, silica, silica-alumina, magnesia,silica-magnesia, zirconia, silica-zirconia, silica-titania,silica-zirconia-titania, acid metal phosphates, acid metal arsenates,crystalline or amorphous aluminosilicate hydrogen zeolites having silicato alumina ratio of about 4:1 to 100:1 and the like and mixturesthereof. Such catalysts are described in greater detail in U.S. Pat.Nos. 4,088,743 and 4,092,404, each incorporated herein by reference.

The amount of recycle gas combined with the acid gas feed ispredetermined to limit the temperature rise in catalytic reactor 14 to atemperature of 1000° F. or less, preferably 850° F. or 700° F. or less.To accomplish the temperature limitation, the amount of recycle gas willrange from about 0.1 mole or less per mole of acid gas feed up to about10 moles or more per mole of acid gas feed. The amount added will dependupon the hyrogen sulfide content of the acid gas feed with the amount ofrecycle being directly proportional to the hydrogen sulfideconcentration of the acid gas feed. Through recycle of the gas from thecatalytic selective oxidation zone, there is prevented undesiredreactions such as the formation of tarry products from hydrocarbons inthe acid gas feed which tend to deactivate the catalyst by coking aswell as overheating or sulfation reactions. Where the apparatus isconstructed of plain steel, it is preferred to limit temperature rise tobelow about 850° F., preferably below about 700° F., to preventcorrosive conditions. The same is true where the gas contains olefins ofmore than 3 carbon atoms and/or paraffins. Input temperature dependsupon the kindling temperature of the catalyst and is typically in therange of about 270° F. to about 450° F., preferably from about 325° F.to about 400° F. A maximum temperature rise is desired to maximizeconversion of hydrogen sulfide to sulfur and sulfur dioxide. Spacevelocities may range from about 1000 to about 5000 or more volumes pervolume of catalyst per hour.

The amount of oxygen combined with the feed gas may range from about 70to about 130 percent of the stoichiometric amount required for the netreaction:

    H.sub.2 S+1/2O.sub.2 →S+H.sub.2 O

although a stiochiometric amount is preferred.

The reactions believed to occur include:

    2H.sub.2 S+1.5O.sub.2 →H.sub.2 O+SO.sub.2

    2H.sub.2 S+SO.sub.2 ⃡(3/2x)S.sub.x +2H.sub.2 O

In any event, any excess oxygen will generally be consumed and theproduct gas of catalytic conversion zone 14 will include hydrogensulfide, sulfur dioxide and sulfur. Water of reaction will be present.

After passing through reactor 14, the gas stream is passed via line 24to condenser 16, where by cooling, the sulfur formed in reactor 14 iscondensed. The residual gas containing hydrogen sulfide and sulfurdioxide and any entrained sulfur is split. A portion is passed by line26 to compressor 28 and combined with the acid feed gas. The remaindermay, depending on pollution regulations, be simply incinerated,subjected to a tail gas treatment such as described in U.S. Pat. No.3,752,877, incorporated herein by reference, or as shown, passed by line30 through indirect heat exchanger 32 where it is heated to the kindlingtemperature of the catalyst contained in reactor 36 and passed by line34 to second catalytic reactor 36. Second catalytic reactor 36 containsa conventional Claus catalyst such as alumina or bauxite, with aluminabeing preferred. In the second catalytic reactor 36, hydrogen sulfideand sulfur dioxide in the gas stream react to form additional sulfur bythe conventional Claus reaction:

    2H.sub.2 S+SO.sub.2 →3S+2H.sub.2 O

Inlet temperatures range from about 350° F. to about 400° F. The productgas is passed by line 38 to condenser 40 where the formed sulfur iscollected. Depending upon the degree of conversion achieved, the gasstream from condenser 40 may be passed to tail gas treatment or reheatedand passed to one or more additional Claus conversion stagescorresponding to catalytic reactor 36, with intermediate condensation ofsulfur and reheating of the gas stream prior to passage to the nextcatalytic conversion stage.

In the process sequence, a single heat transfer fluid such as steam,Dowtherm™ manufactured by Dow Chemical Co., Therminol™ manufactured byMonsanto, Mobiltherm™ manufactured by Mobile Oil, and the like, may beused effectively for complete control of the process. A fluid whichremains liquid throughout the process with a low enough vapor pressureto operate as close as possible to atmospheric is preferred. Suitablefluids include high-boiling Dowtherm™, a mixture of diphenyl anddiphenyl oxide, Mobiltherm™600, an aromatic mineral oil, Therminol™55, aclear yellow synthetic hydrocarbon mixture boiling in the range of 635°F. to 734° F., Therminol™60, a poly aromatic compound boiling in therange of 550° F. to 741° F., Therminol™66, a modified terphenyl, and thelike. The preferred heat transfer fluids are non-aqueous. The heattransfer sequence shown is for use of a single fluid which substantiallyremains liquid.

The heat exchange fluid is pumped from storage tank 42 through line 44and split. A portion is passed by line 46 to indirect heat exchanger 16and the balance by line 48 to cooler 40. The effluent from indirect heatexchanger 16, which receives the heat of cooling and condensation ofsulfur, leaves heat exchanger 16 by line 50 and is again split. Aportion is passed by line 52 to indirect heat exchanger 18 for heatingthe feed gas stream, and the balance by line 54 to indirect heatexchanger 32, which provides heat to the gas passing from indirect heatexchanger 16 to catalytic reactor 36. The effluents from indirect heatexchanger 40, exiting by line 56, indirect heat exchanger 32, exiting byline 58, and indirect heat exchanger 18, exiting by line 60, arecombined in line 62 for passage through trim cooler 64 for return toreservoir 42. Indirect gas-fired heat exchanger 66 is employed forstartup operations and, if necessary, to provide auxiliary heat for theprocess.

Any tail gas may be treated for recovery of sulfur values, preferably inaccordance with U.S. Pat. No. 3,752,877 incorporated herein byreference.

EXAMPLE 1

A feed of 357.29 moles per hour of an acid gas containing about 88.18moles H₂ S, 22.66 moles H₂ O, 243.22 moles CO₂, 1.36 moles C₂hydrocarbons, 0.41 mole C₂ hydrocarbons, 1.17 moles C₃ hydrocarbons,0.25 mole NC₄ hydrocarbons and 0.04 mole NC₄ hydrocarbons at a pressureof 24.70 psia and at a temperature of 110° F. is combined with about2145 moles per hour of the effluent of condenser 16 as recycle gas andhaving the approximate composition shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Component     Moles/Hr.                                                       ______________________________________                                        H.sub.2 S     40.34                                                           SO.sub.2      20.18                                                           H.sub.2 O     434.53                                                          O.sub.2       0.00                                                            N.sub.2       663.30                                                          S.sub.2       .00                                                             S.sub.4       .00                                                             S.sub.6       .18                                                             S.sub.8       .20                                                             CO.sub.2      972.75                                                          H.sub.2       0.00                                                            CO            0.00                                                            COS           .17                                                             CS.sub.2      0.00                                                            C.sub.1       5.44                                                            C.sub.2       1.64                                                            C.sub.3       4.68                                                            NC.sub.4      1.00                                                            NC.sub.5      .16                                                             ______________________________________                                    

The effluent recycle gas temperature is about 350° F. and the pressureis 22.1 psia. Air at 180° F. in an amount of 216.34 moles per hour iscombined with the mixture of acid gas and recycle gas and fed to reactor14 containing Selectox-32, a selective oxidation catalyst available fromUnion Oil Company of California, at a temperature of about 375° F. andat a pressure of 22.0 psia. The exit gas leaves reactor 14 at atemperature of about 670° F., a pressure of about 21.31 psia, at a rateof about 2692 moles per hour and has the approximate composition shownin Table 2.

                  TABLE 2                                                         ______________________________________                                        Component     Moles/Hr.                                                       ______________________________________                                        H.sub.2 S     50.18                                                           SO.sub.2      25.09                                                           H.sub.2 O     542.01                                                          O.sub.2       .00                                                             N.sub.2       829.07                                                          S.sub.2       1.30                                                            S.sub.4       .05                                                             S.sub.6       9.77                                                            S.sub.8       1.83                                                            CO.sub.2      1215.93                                                         H.sub.2       0.00                                                            CO            0.00                                                            COS           .20                                                             CS.sub.2      .00                                                             C.sub.1       6.80                                                            C.sub.2       2.05                                                            C.sub.3       5.85                                                            NC.sub.4      1.25                                                            NC.sub.5      .20                                                             ______________________________________                                    

After sulfur condensation in condenser 16, about 80 percent of the gasis recycled to reactor 16 and the balance fed at a temperature of 430°F. to a Claus reactor containing an alumina catalyst. Sulfur in theeffluent of the Claus reactor is condensed at 350° F. to give an overallsulfur recovery of about 92 percent before tail gas treatment to recoverresidual sulfur.

EXAMPLE 2

Example 1 is repeated except that sulfur is condensed after the Clausreactor at 280° F., for an overall recovery of 94.86 percent before tailgas treatment.

EXAMPLE 3

Example 1 is repeated except two Claus conversion stages are employedfollowing the selective oxidation stage to give an overall sulfurrecovery of 96.67 percent prior to tail gas treatment.

What is claimed is:
 1. A process for the production of sulfur withcontrol of process temperature by use of a liquid heat transfer fluidwhich comprises:(a) providing a closed loop heat transfer systemcontaining a liquid heat transfer fluid; (b) condensing sulfur fromproduct gas stream containing sulfur, sulfur dioxide, and hydrogensulfide by indirect heat exchange with cooled liquid heat transfer fluidstream in a first indirect heat exchange zone to provide a cooledresidual gas comprising hydrogen sulfide and sulfur dioxide and a heatedliquid heat transfer fluid stream, said product gas stream being formedby contacting, in a catalytic selective oxidation zone, a feed gasheated, at least in part, for feed to said catalytic selective oxidationzone by indirect heat exchange with a first portion of the heated liquidheat transfer fluid stream in a second indirect heat exchange zone, saidfeed gas comprising an acid gas feed containing 10 percent and up to 100percent by volume hydrogen sulfide in admixture with oxygen present inan amount of from about 70 percent to 130 percent of the stoichiometricamount required for oxidation of the hydrogen sulfide to elementalsulfur and a recycle gas which is a portion of the cooled residual gascomprising hydrogen sulfide and sulfur dioxide exiting the firstindirect heat exchange zone, with a selective oxidation catalyst capableof selectively oxidizing hydrogen sulfide to sulfur dioxidesubstantially without formation of sulfur trioxide and catalyticallyconverting introduced hydrogen sulfide to sulfur dioxide and sulfur at atemperature above the kindling temperature of the catalyst and below anupper reaction temperature of about 1000° F., said upper reactiontemperature being selectively controlled by the amount of recycle gasintroduced to the catalytic selective oxidation zone; (c) indirectlyheating the balance of cooled residual gas exiting the first heatexchange zone in a third indirect heat exchange zone with a secondportion of the heated liquid heat transfer fluid stream to a temperaturefor feed to a Claus conversion stage; (d) passing the heated balance ofthe residual gas stream to at least one Claus conversion zone to formadditional sulfur; and (e) combining liquid heat transfer fluid streamsfrom the second and third indirect heat exchange zones and cooling thecombined liquid heat transfer fluid streams for recycle to the process.2. A process as claimed in claim 1 in which a portion of the cooledliquid heat transfer fluid is utilized to condense sulfur from theeffluent of at least a first Claus conversion zone following thecatalytic selective oxidation zone by indirect heat exchange in a fourthheat exchange zone with the effluent of said first Claus conversion zoneand combining the liquid heat transfer fluid from the fourth heatexchange zone with the liquid heat exchange fluids from the second andthird heat exchange zones.
 3. A process as claimed in claim 1 in whichthe upper reaction temperature is below about 700° F.